Documentation: Update macOS development instructions

[coldjugular]

Update the macOS local development instructions to reflect the development flag.

Checklist

• [ ] I've run tests to see all new and existing tests pass
• [ ] I've followed the code style of the rest of the project
• [x] I've read the Contribution Guidelines
• [x] I've updated the documentation if necessary

Note: I haven't run the tests since this only changes the README.

If you've made changes to gyb files

• [ ] I've run .script/generate_boilerplate_files_with_gyb and included updated generated files in a commit of this pull request

Motivation:

The README instruction was out of date.

Modifications:

Update the macOS local development instructions to correspond to the changes to Package.swift made in commit 17d95a67829a19d06da3857d4d66cbfa120b3d21

Result:

[Lukasa]

@swift-server-bot test this please